It has been known to modify nucleosides, nucleotides, and certain oligonucleotides for various purposes. Included among such known modifications are modifications to the groups linking the sugar moieties of said nucleic species. Thus, it has been known to alter the phosphodiester bonds naturally extant in nucleic acids to provide what has been perceived to be improved structures, especially structures which have improved cell uptake. A number of such modifications are known including phosphothioates, substituted phosphonates and others. The general synthetic scheme for arriving at such analogs has been to involve the 5'-hydroxyl group of a nucleoside or its nucleotide, either bound to a polymeric carrier or to a sequence-specified 3'-nucleotide with its phosphorus atom in either the pentavalent or trivalent oxidation state. Specific coupling procedures have been referred to as the phosphorus triester, the phosphorus diester, the phosphite triester, and the hydrogen phosphonate phosphorylation procedures. Commercially available monomers and polymeric carrier-bound monomers are available for such methods having protective basis (G, A, C, T, U and other heterocycles) along with protected phosphorus atoms to allow storage and prevent non-specific reactions during the coupling process. Catalysts for enhancing the electrophilicity of the 5'-hydroxyl group are not required but are available.
Synthesis of non-ionic methyl phosphonates and ionic phophorothioates are similarly known and both oligonucleotide analog classes are currently receiving attention as gene modulating agents. Such prior attempts at modifying the intersugar linking groups have found some promise in therapeutics and the like however each exhibits substantial shortcomings. Thus, with linkages such as methyl phosphonate diester linkages, chirality is introduced into the system. Since the different forms of such chiral materials are generally absorbed into cells at different rates, the different forms of such materials are believed to lead to less-than-optimum performance.
Other materials, including the naturally-occurring phosphodiester forms, exhibit an ionized condition which is believed to interfere with cell absorption. It is believed that substantially non-ionic materials will be absorbed more readily by cells and be more effective in therapeutics and the like.
Both the methyl phosphonate and phosphorothionate modifications of oligonucleotides are believed to impart nuclease resistance, to enhance to some degree cellular transport of oligomers and to strengthen hybridization binding of the oligomer to target nucleic acid sequences. However, still greater improvement in these qualities is required before effective therapeutics, diagnostics, and research tools become available. Accordingly, there is a long-felt need for improved oligonucleotide analogs, for corresponding component nucleosides, and for compositions useful for the formulation of oligonucleotide analogs which, at once, are substantially non-chiral and non-ionic. Such materials, which are provided in accordance with the present invention, are believed to lend superior qualities of cell uptake, nuclease resistance, and improved hybridization with target RNA.